Gandalf:
Having had a child in two years in a nursery setting where there was no direction whatsoever other than handing my child a pot of pencils or a pot of paint, simply leaving them to it does not work at this stage.
What does 'does not work' mean?
What do you understand to be the purpose of the exercise?
We had two years of brown splodge. Even my DD did not know what she had painted. She did not learn techniques such as drawing animals, people, buildings or anything else. She was not painting or being creative being left to it. She was just putting paint on paper.
That is what she was supposed to be doing - sensing the paintbrush moving across the paper, changing direction if the fancy struck her, feeling the difference between very watery paint and thicker paint as she applied it, watching how the paint changed colour as it dried. Painting is a sensory experience every bit as much as it is an expressive exercise in nursery and up to age 7ish.
The form of the end product is completely irrelevant for children this age. What they are doing at this young age is expressing exuberance or other feelings, not trying to faithfully depict a particular topic. Expression has nothing to do with logic, scale or form. It's all about emotion.
They will all develop to the stage of trying to produce more representational work at the right time for them.
The most a parent can or should directly do for young children when it comes to art is teach names for colours, provide lots of materials for self expression and encouragement to explore (i.e. not too much focus on avoiding a mess though obv most parents are going to keep their child aware of the importance of confining their self expression to paper or blackboards, etc.). You can show them how to clean up (and help them), ask them what side of their work is 'up', ask them to tell you what they were thinking about when they were doing it (not the same as asking them what it is).
Children need exposure to lots of sensory experiences at this age, and if you encourage them they will process that experience via art when they have (for instance) come home from an hour of jumping in puddles or running in the park or helping grandad in the garden. Or a trip to a beach. Don't expect a recognisable visual reproduction of any scenes from those activities. It's a good idea to stick to recent activities as inspiration for topics, though some children will be very self directed, and some will focus on themes that are particular to them.
Another element of the nursery scene the OP described that bothered me was that 'beach' may not have been a recent experience for the children, unless they all live right beside one. Some children in the nursery may have never visited a beach. 'Breakfast' might have been a better topic (though of course that may be a non-event for some children too, sadly).
A depiction of 'a beach' will feature what the child aged 4-7 feels is most important about 'a beach'. Younger children will express the topic in similar vein. Up to age 7 a child is in a 'preschematic' intellectual phase where visual depiction is concerned. It's an important phase in the development of the intellect and should not be cut short by attempts to teach 'the correct' or 'better' ways to depict topics. Colour choice, dimensions of objects that are depicted and form of objects (if any) are all done with no regard for logic; there is no aim of reproducing a reasonable facsimile of a topic, an existing picture or any sort of template. That comes much later, at age 9 and above. What the child puts on paper at nursery age is an emotional snapshot, a representation of the world that comes from his 'mind's eye' that is still developing. It is really important not to interfere with the process of expressing that.